The present invention relates to a control valve for controlling a fluid passage, and more specifically to a control valve effectively used as an fluid pressure passage control valve, adapted to open and close a passage for fluid pressure supplied from a master cylinder to, for example, wheel cylinders of a braking system of an automobile provided individually for wheels of the vehicle, and capable of executing braking force control, such as antiskid control, with high response characteristics.
In a conventional braking system of an automobile, a master cylinder produces an fluid pressure in response to a force with which a brake pedal is worked, and the pressure is transmitted to wheel cylinders arranged individually for wheels of the vehicle. In each wheel cylinder, a brake piston is driven by the supplied fluid pressure so that a braking force responsive to the working force on the pedal acts on each corresponding wheel.
In performing a braking operation on the braking system constructed in this manner, if a strong braking force is applied to a wheel, the wheel may sometimes lock and slip on a road surface. In such a case, it is necessary to execute antiskid control such that the slip between wheel and road surface is quickly removed to increase the contact resistance between them, thereby ensuring stable driving of the vehicle. The antiskid control may be effected by manually adjusting the working force on the brake pedal. It is desirable, however, that the braking force on the slipped wheel should automatically be reduced when a locked state of the wheel is detected.
A system for such automatic antiskid control has conventionally been proposed. In this system, a valve mechanism is provided in each of hydraulic circuits arranged individually between a master cylinder and wheel cylinders, and the braking force is reduced by discharging braking fluid from that wheel cylinder which corresponds to the slipped wheel.
Usually, a solenoid-operated valve is used as a fluid control valve for operating the hydraulic circuit. In this case, the fluid pressure passage is opened and closed electrically. In the solenoid valve, however, a solenoid coil for supplying exciting current has an inductor, so that the valve plug is actuated with a delay after the solenoid coil is energized. Thus, it is hard to obtain high response characteristics suited to antiskid control.
In consideration of these circumstances, piezoelectric elements, as a high-response drive source, may be used in place of the solenoid mechanism. The coefficient of thermal expansion of these elements is as low as 1.times.10.sup.-6 /.degree. C., while those of a mechanism for supporting the elements and other members constituting the control valve range from 12.times.10.sup.-6 /.degree. C. to 23.times.10.sup.-6 /.degree. C. Thus, there is a considerable difference between these values.
When using the control valve including the piezoelectric elements, therefore, if the temperature of the control valve section or that of a fluid as an object of control greatly changes, a difference in thermal displacement related to thermal expansion or contraction is caused between the piezoelectric elements and other members. Thus, even with the same control instruction, the opening of the valve plug of the control valve varies with temperature conditions. It is therefore difficult to constantly maintain satisfactory conditions for high-accuracy control.
More specifically, when using the control valve with the piezoelectric elements as its drive source in a motor vehicle, the working temperature of the valve widely ranges from -30.degree. to +120.degree. C. Accordingly, the control valve cannot easily maintain its normal functions.